1. Field of the Invention
The present invention relates to the technique of controlling a motor inverter for supplying a current to a motor for each phase.
2. Description of the Related Art
Generally a motor inverter is used to drive a motor such as a 3-phase motor etc. to which a current is supplied for each phase. The motor inverter is normally provided with two switching elements (a total of six switching elements) for each phase. Thus, a rotating magnetic field is generated for the motor by allowing the six switching elements to perform an ON/OFF switching operation and supplying a current to each phase through the switching elements. A control device of the motor inverter controls the switching operation of the switching elements of the motor inverter, and drives the motor.
The switching operation of the switching elements prepared for each phase is performed for each area by dividing one rotation (360 degrees) of the rotor of the motor into a plurality of areas. Thus, the drive signal provided for the switching operation of each switching element is determined for each area.
Driving a motor using a motor inverter is accompanied with a switching loss of the switching operation of a switching element. A method of reducing the switching loss can be a 2-phase modulation method. In the 2-phase modulation method, the switching operation of switching elements is not permitted in one phase of the three phases. That is, two switching elements are placed in a stationary phase in which they are placed in an ON or OFF position, and the switching elements are allowed to perform the switching operation in the remaining two phases. The stationary phase is changed for each area. Thus, by setting a stationary phase for each area, the switching loss can be reduced.
The conventional 2-phase modulation method is disclosed by, for example, Japanese Laid-open Patent Publication No. 8-340691 (U.S. Pat. No. 5,699,240, hereinafter referred to as the “patent document 1”). In the conventional 2-phase modulation method described in the patent document 1, a stationary phase is selected and determined by the phase difference between the current command value of each phase and the voltage command value of each phase obtained from the current command value of each phase.
The switching loss is a loss of power. Thus, to suppress the switching loss, it is preferable that the phase in which the maximum power (maximum power phase) is acquired is set as a stationary phase. Therefore, although the 2-phase modulation method is adopted, it is also important to more efficiently suppress the switching loss.